The present invention relates to processors of film and similar photosensitive media, in general; and, in particular, to a processor having means to vary transport speed and including means to maintain fixed interfilm spacing regardless of transport speed.
Photosensitive media processors, such as the Kodak X-OMAT processors, are useful in applications such as the automatic processing of radiographic films for medical imaging purposes. The processors automatically transport sheets or webs of photosensitive film, paper or the like (hereafter "film") from a feed end of a film transport path, through a sequence of chemical processing tanks in which the media is developed, fixed, and washed, and then through a dryer to a discharge or receiving end. The processor typically has a fixed film path length, so final image quality depends on factors including transport speed which determines length of time the film strip is in solution, and the temperature and composition of the processing chemicals (the processor "chemistry").
In a typical automatic processor of the type to which the invention relates, film transport speed is set at a constant rate and the chemistry is defined according to a preset recommended temperature, e.g. 93.degree. F., with a specified tolerance range of +/-X.degree. F. A temperature control system is provided in the processor to keep the chemicals within the specified range.
Conventional processors usually include a film width sensor in the form of a reflective infrared sensor array adjacent a feed entrance opening, and may also include a feed detector in the form of a Hall effect switch or the like for detecting separation of entrance rollers due to the passage of film sheets at the front end of the transportation path. The film width sensor not only provides an indication of the width of a sheet entering the processor, but may also provide an indication of the occurrence of the leading edge and trailing edge of each sheet, since the signals from the film width sensor will change significantly as each leading and trailing edge is encountered. Information as to leading and trailing edge occurrences and width of the film, taken with prior knowledge of the constant transport speed, is used to keep track of cumulative total film surface area processed in order to guide chemistry replenishment control. The use of a separate entrance roller detector signals that a sheet of film has actually entered the nip of the first roller pair, and is not just sitting still on the film guide under the width sensor.
When sheets of film are sequentially fed into a processor, it is desirable that a spacing be maintained between the trailing edge of a first sheet and a leading edge of a next one in order to avoid overlap. The spacing should be enough so that overlap is avoided even though one or both of the sheets suffer some slippage and/or skewing along the transport path, but not so great that processing time is unduly affected. For a particular constant fixed transport speed processor, proper film spacing may be controlled by a fixed set time interval between the entry into the processor of the trailing edge of the first sheet and the time when the user is signalled to enter the next sheet. When the entry of the leading edge of the first sheet into the processor is detected, an annunciator in the form of a "wait" light is illuminated to signal that the required spacing has not yet been attained. At a set time after the entrance of the trailing edge into the processor has been detected, the "wait" light is extinguished and a "ready" light is illuminated.
Although conventional processors used for radiographic image processing are traditionally configured to operate at a constant film transport speed, modifications may be made through gear changes and the like to vary the process. Moreover, new processors are being introduced which are usable in more than one mode. The mode is often referred to in shorthand fashion by a nominal film transport "drop time", which may be defined as the time from entry of the leading edge of a sheet of film at the feed end until exit of the trailing edge of the same sheet of film at the discharge end. Conventional processors operate in standard (90 second), rapid (45 second), or "Kwik" (30 second) mode, and can be varied to operate in an extended-cycle mode, such as described in L. Taber & A. G. Hans, "Processing of Mammographic Films: Technical and Clinical Consideration," Radiology, Vol. 173, No. 1, pages 65-69, Oct. 1989. In the latter mode, processor speed is lowered and chemistry temperature is raised to enhance image contrast for better detection of changes in density of fibrous tissue. The new processors will be settable as to run parameters, including transport speed in order to be able to use the same processor for multiple processing modes.
It is desirable, in a processor having selectable transport speed, to be able to maintain a fixed interfilm spacing during transport regardless of the transport speed setting. Conventional systems that operate on a fixed time interval to determine film spacing are inadequate for this purpose. When faster transport speeds are selected, the same fixed time interval will give a spacing that is too great. When slower transport speeds are selected, the same interval will give a spacing that is too small.
One prior art arrangement, described in U.S. Pat. No. 4,300,828, sets spacing using a feed counter clocked by a drive shaft encoder. A microcomputer loads the feed counter with a number corresponding to the number of shaft encoder pulses needed to drive the trailing edge of the sheet past a particular point of the transport path. When the count reaches zero, a feed annunciator is actuated and a "wait" light is turned off. There is no teaching or recognition in the '828 patent of using such a shaft encoder system in a processor whose transport speed is settable. Moreover, the mechanical nature of the encoder limits programming flexibility.